Production of interferon

ABSTRACT

The present invention relates to a process to isolate genetic material (DNA) containing the nucleotide sequence coding for interferon in human fibroblastic cells which comprises cultivating cells producing interferon when exposed to an inducer of interferon, exposing same to such inducer, extracting messenger RNA from said induced cells, purifying the interferon messenger RNA, transcribing the messenger RNA into DNA and cloning the DNA in a suitable vector. Preferred cells are human diploid foreskin cells. The invention further relates to a process for engineering a bacterial strain to produce interferon polypeptide which comprises introducing a cloned interferon DNA into a suitable vector-carrier. A preferred vector-carrier is E. coli. The invention also relates to the mRNA of human interferon in highly purified form, to the mRNA of human interferon in β1 highly purified form, to the mRNA of human interferon in β2 highly purified form, to the DNA coding for a polypeptide having interferon activity, insertable in a vector, such as plasmid pBR322, and also to human interferon β1 in highly purified form, and human interferon β2 in highly purified form.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.06/208,925, filed Nov. 20, 1980.

BACKGROUND OF THE INVENTION

Interferon is an important antiviral and antitumor protein produced bythe body. Because of its species specificity, clinical use of interferonrequires human interferon. The limited amount of interferon that can beproduced from tissue culture cells or from fresh white blood cells, isnot sufficient for large-scale clinical use. Introduction of the geneticinformation for human interferon into a bacterial microorganism couldpossibly, if it were available, allow the mass-production of apolypeptide having interferon activity. It is known that such techniqueshave been developed for human growth hormone and insulin.

The two types of interferon, i.e. leucocytes (IFN-α) and fibroblasttypes (IFN-β) appear to be coded by different messenger RNAs (Cavallieriet al., 1977, Proc.Natl.Acad.Sci.USA 74, 4415-19). Isolation of thesemRNAs in pure form has not been achieved as yet. This appears to be allthe more difficult as the host cell is liable of synthesizing theinterferon mRNA only as a result of exposure of the latter to suitableexogenous factors, for instance, viral infection or particularexperimental polynucleotides such as poly (rI:rC), and only in minuteamounts. Accordingly, if it had already been suggested to use a mRNAextract of host cells which had preliminarily been induced to produceinterferon and to cause it to be translated in vitro in a cell-freesystem comprising all ingredients, particularly the natural aminoacids,whereby a protein preparation having interferon activity had beenobtained, the interferon mRNA was indeed but a very minor proportion ofthe mRNAs being translated and accordingly the final protein preparationhaving interferon activity was more than highly contaminated with otherproteins.

Therefore, the use of the mRNA preparations of the prior art as astarting material for direct transforming into double stranded DNAliable of being cloned after its insertion in a suitable vector, wouldinvolve screening difficulties which would be insuperable in practice.

OBJECTS OF THE INVENTION

A major object of the present invention is to overcome to the greatestextent the above difficulties, particularly of providing a method ofisolating the mRNA translatable into interferon, more particularly ofhuman origin as well as the corresponding DNAs.

It is a further object of the present invention to provide a process forthe isolation of genetic material (DNA) containing the nucleotidesequences coding for interferon in human cells.

It is yet a further object of the present invention to provide a processof this type wherein the messenger RNA is transcribed into DNA and whichcomprises cloning the DNA in a suitable vector. A preferred vector is aplasmid, and a preferred plasmid is a plasmid of the pBR322 type.

Furthermore it is an object of the present invention to provide aprocess to detect the different genetic sequence expressed in humancells when it is induced to produce interferon. A preferred embodimentcomprises effecting a differential hybridization of the bacterial clonesfirst with DNA from induced-cells RNA and second with identical DNAderived from uninduced-cell RNA.

It is yet a further object of the present invention to provide a processof engineering a bacterial strain to produce interferon polypeptidewhich comprises introducing a cloned interferon DNA into a suitablevector-carrier. The cloned interferon DNA is advantageously produced asset out above. A preferred vector is E. coli or another suitablemicroorganism. Another type of preferred vector-carrier is a eukaryoticcell.

Other and further objects of the present invention will become apparenthereinafter.

SUMMARY OF THE INVENTION

A preferred aspect of the present invention comprises a process toisolate genetic material (DNA) containing the nucleotide sequence codingfor interferon in human cells, preferably fibroblastic cells, whichcomprises cultivating cells producing interferon when exposed to aninducer of interferon, exposing same to such inducer, extractingmessenger RNA from said induced cells, purifying the interferonmessenger RNA, reverse transcribing the messenger RNA into DNA andcloning the DNA in a suitable vector. According to a preferredembodiment the cells are human diploid foreskin cells. According to apreferred embodiment the inducer is double-stranded RNA.

The process according to the invention of enrichment of interferon mRNAor of a mRNA (inducible mRNA) of other protein or polypeptide theproduction of which is liable of being induced in the host cell byexogenous factors comprises:

a) exposing a culture of the host cell to such exogenous factor toinduce in said host cells the synthesis of the said inducible mRNA;

b) extracting the mRNAs including the inducible mRNA formed in saidinduced cell culture therefrom as well as the mRNAs of a non-inducedcontrol culture of the same host cells;

c) synthesizing cDNA probes of the mRNAs of both the induced culture andthe non-induced control culture using the corresponding mRNAs astemplates (induced cDNAs and non-induced cDNAs);

d) synthesizing double-stranded cDNAs derived from the mRNA extractedfrom the induced culture, inserting said cDNAs in appropriate vectors,transfecting suitable microorganisms with the modified vectors obtainedand cultivating said microorganisms under conditions suitable to causeselective development of microorganism colonies of said modified vectors(initial colonies);

e) forming duplicate colonies of said initial colonies;

f) causing freeing in situ of the DNAs of both said initial andduplicate colonies;

g) hybridizing the DNAs of the initial colonies on the one hand and theduplicate colonies on the other hand respectively with the abovesaidinduced cDNA probes and non-induced cDNA probes (or conversely);

h) recovering DNAs of clones which hybridize with the induced cDNAprobes and do not hybridize with the non-induced cDNA probes, wherebyDNAs hybridizable with the mRNA capable of being translated into saidinducible protein or polypeptide, particularly interferon, are obtained.

The process of the invention is preferably applied to the production ofhighly-purified mRNA of interferon of human origin.

It will be appreciated that some of the steps defined above need not beperformed in the exact order which has been indicated hereabove. Thisapplies particularly to step c) relating to the synthesis of the cDNAprobes. In fact the latter step c)even needs not be a step of theprocess of the invention per se. Analogous probes, from other cellularsources may be used instead.

In a further advantageous embodiment of the invention, steps c)and d) ,as above defined, are carried out on fractions only of the mRNAs whichcan be extracted from cells, whether they have been "induced" or"non-induced". In that respect advantage can be taken of the fact thatmRNAs comprise a poly-A portion which enables the separation of the mRNAfrom total RNA by binding to oligo-T-cellulose. The subsequently elutedmRNA fraction is advantageously subsequently fractionated by sucrosegradient centrifugation, the different bands being then separatelytranslated into a suitable cell-free system, such as a reticulocytelysate, each of the translation products being then tested for theircapability of being immunoprecipitated by an anti-interferon serum. Thebands of mRNA whose translation products gave a positive reaction insuch immunoprecipitation test were then retained for carrying out thetwo above defined steps c) and d). A further aspect of this procedure isthat two different mRNAs coding for interferon can be isolated fromhuman fibroblastic cells if these cells are induced as in a) above. Thesmallest mRNA sediments at 11S and yields by translation in a cell-freesystem a protein of molecular weight 20,000 which is selectivelyprecipitated by antibodies prepared against one of the interferons thatcan be purified from these cells. This protein is human interferon(IFN)-β1. The largest mRNA sediments at 14S and yields a protein ofmolecular weight 23,000 which is precipitated by antibodies against aless purified preparation of fibroblastic interferon. This protein isdesignated human interferon-β2. Fractions of mRNA for both proteins wereused for step d) above. This makes possible to produce at will IFN-β1 orIFN-β2 in substantially pure form. Hybridization (steps c, d, e, f, g,and h defined hereabove) thus unable single colonies of E. colicontaining one of two interferon cDNA IFN β1 or β2 to be exactlyidentified. Particularly, it will be appreciated that those colonieswhich hybridize only with "induced" cDNA probes and not with the"non-induced" cDNA probes can only consist of those which haveincorporated a modified vector having the cDNA corresponding to theinterferon mRNA, inasmuch as: a) the said DNA, not being hybridizablewith the "non-induced" cDNA probe, does not accordingly correspond toany of the mRNAs normally produced by a "non-induced" cell and b) theonly difference between the two probes being used is that the "induced"cDNA probe differs only from the other one by the fact that it containsa cDNA derived from one of the interferon mRNA. It will be obvious tothe man skilled in the art that the forming of these cDNA probes can beachieved by any method known per se, particularly by transcription inthe presence of a reverse transcriptase. It is of advantage to use ahigh producer bacterial vector such as the pBR322 plasmid. In order toobtain the later expression in a microorganism of the vectors modifiedby the interferon cDNA, one may resort to a set of vectors as defined byPatrick Charney et al., in "Bacteriophage Lambda and Plasmid VectorsAllowing Fusion of Cloned Genes in each of the three translationalphases". Nucleic Acids Res., 1978 - 5(12), 4 479-94.

According to a further important step of the invention, the abovemodified vector, whichever be its translational phase, can be used forextracting interferon mRNA from the RNA mixture produced by "induced"cells, which process comprises contacting these RNAs with such a vectormodified by the interferon cDNA previously immobilized on a support,under conditions suitable for causing hybridization and thereaftereluting the fixed mRNA from the immobilized DNA vector. In such a way,mRNA of human interferon (either IFN may be obtained in highly purifiedform.

The high state of purification can be appreciated by the fact that thetranslation product, in a suitable in vitro system, consists in etchcase essentially of a single polypeptide compound having interferonactivity, and which can be precipitated by said antibodies to humaninterferon.

The invention thus concerns also said purified mRNAs which comprisesnormally up to 900-1,000 nucleotides for IFN-β1 and 1,250-1,350 forIFN-β2. In the same manner it also concerns the corresponding cDNA whichcan be obtained by reverse transcription of said RNAs. The inventionconcerns also the existence of two different interferon mRNAs and henceprotein moieties that may have also different biological importance. Itgoes without saying that each of the hybridization steps involved in thepresent application will be preceded, where appropriate, by adenaturation of the possible double-stranded nucleic acids to insurethat no double-stranded nucleic acids (which could induce interferonactivity in cells) remains in the purified mRNAs when these are used forthe production, by translation thereof, of substantially pure proteinhaving interferon activity in vitro.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention will be further illustrated in a non-limitative manner bythe more detailed description of a preferred embodiment thereof, takinginto account, the appropriate Figures and Table.

BRIEF DESCRIPTION OF THE FIGURES AND TABLE

In the enclosed figures:

FIG. 1 illustrates the fractionation of mRNA on a sucrose gradient andthe translation of these mRNA fractions originating from induced cellsto produce human interferon activity in Xenopus laevis oocytes and toproduce specifically immuno-precipitated proteins in reticulocytelysates, a process which separates the mRNA for IFN-β1 and IFN-β2.

FIG. 2 illustrates the result of a differential hybridization procedureof the DNAs of the same bacterial colonies with the two abovementionedprobes from "induced" and "non-induced" cells, respectively.

FIG. 3 illustrates a purification of interferon IFN-β2 mRNA byhybridization to immobilized DNA from bacterial clone A341 asdemonstrated by translation in a reticulocyte lysate.

FIG. 4 demonstrates that DNA from bacterial clone A341 is complementaryto a 1,250-1,350 long mRNA which appears in human cells only afterinduction of interferon synthesis.

Table 1 demonstrates that this mRNA, upon translation in Xenopus laevisoocytes, yields biologically active interferon which inhibits the growthof a virus in human cells.

a) Purification of two interferon mRNAs from human diploid fibroblasts

RNA was extracted from monolayer cultures of the human fibroblast lineFS11 (isolated at the Weizmann Institute of Science). These diploidcells grown from foreskin explants taken from a normal individual 8 daysafter birth, were selected among 15 separate isolates for their capacityto produce high titers of interferon. Alternatively cultures of a cloneof human SV80 cells were used. The cultures in Eagle's minimal mediumwith 10% fetal calf serum were maintained in 2.2 liter glass rollerbottles or 22×22 cm plastic trays in 5% CO₂, 95% air at 37° C. Threedays after confluency, the cultures were induced to produce interferonby exposure to poly(rI):(rC) 100 μg/ml, and cycloheximide (which blocksthe synthesis of proteins by the host) 50 μg/ml for 3.5 hours.Actinomycin D (which blocks the synthesis of cellular RNA) 1 μg/ml, wasadded and 1 hour later the cells were lysed with buffered Nonidet-P40detergent and cytoplasmic RNA was extracted with a phenolcresol mixtureas Kirby (1965 ). The mRNAs were isolated from total RNA, by bringinginto play the fact that they contain Poly A, by binding tooligo-dT-cellulose. The mRNA fraction was subsequently fractionated bysucrose gradient centrifugation. The fractions containing interferonmRNA were identified by micro, injection to Xenopus laevis oocytesaccording to Raj N. K. B and Pitha P. M. (Proc. Natl.Acad. Sci. USA 74,1483-1487, 1977), and measuring 24-40 hours later the antiviral activityof the interferon released in the oocyte incubation medium. Antiviralactivity was measured by exposing FS11 cells to dilutions of the oocytemedium, infecting said cells with vesicular stomatitis virus andobserving inhibition of the cytopathic effect caused by the virus.Interferon titers were calculated by comparison to a known solution,according to the last effective dilution. The fractions containinginterferon mRNA were also identified by translation in a reticulocytelysate followed by immunoprecipitation of the product according to themethod of Weissenbach et al., (Eur. J. Biochemistry 98, 1-8, 1979).

FIG. 1a shows the two peaks of interferon mRNA activity detected byinjection to oocytes. FIG. 1b is representative of theimmunoprecipitation lines obtained between the translation products andanti interferon serum, the two arrows showing the two polypeptides ofmolecular weight of 23,000 (23K) and 20,000 (20K). The sucrose gradientfractions coding for the 23K and 20K immunoprecipitated polypeptides areshown in FIG. 1a and can be seen to correspond to the two peaks ofinterferon mRNA activity. Interferon activity was also detected in thetranslation products of reticulocyte lysates by measuring induction ofthe (2'-5') oligo-isoadenylate synthetase in human cells. By bothmethods it was seen that the largest interferon mRNA peak codes for the23K polypeptide, while the smallest interferon mRNA peak codes for the20K poiypeptide. Both interferon mRNAs were in this way isolated andused for cloning in E. coli.

b) Cloning of interferon β2 cDNA in E. coli

The purified mRNA from induced cells was calculated to contain about1-3% of the mRNA for the 23,000 MW polypeptide and was used as templateto synthesize cDNA with avian myeloblastosis virus, reversetranscriptase and oligo-dT as primer. After eliminating the RNA byalkali treatment, the second strand of DNA could be synthesized withreverse transcriptase or DNA polymerase I. Single-stranded DNA wascleaved off with nuclease S1, and the 3' ends of the DNA was elongated("tailed") with nucleotide terminal transferase using dCTP as substratePlasmid pBR322 DNA was linearized with restriction endonuclease and wasdG tailed with dGTp. The plasmid DNA was then hybridized with thedC-tailed human cDNA described above, and used to transfect E. coliDP50. Transfected bacterial colonies were identified by plating on agarplate containing Luria broth, diaminopimelic acid, thymidine andtetracycline. The colonies were further tested on similar agar platesbut containing ampicillin as the only antibiotic. The ampicillinsensitive, tetracycline resistant bacterial colonies were grown on anitrocellulose filter deposited on an agar plate as above withtetracycline 10 μg/ml. Over two thousands of the transformed coloniesobtained were respectively transferred in part on other nitrocellulosefilters, themselves on agar plates as hereabove indicated, each of theduplicate colonies being related (particularly by common numbering) toone of the initial colonies. After the colonies reached 3-5 mm indiameter, the filter (initial cultures and duplicates) were transferredon top of a stack of filter papers impregnated first with 0.5N NaOH,then with 0.15M NaCl and 0.1N NaOH to cause release in situ of theirrespective DNAs. The filters were neutralized and dried. To detect thebacterial colonies containing the interferon DNA sequences, the filterswere hybridized with two different [³² P]cDNA probes. One cDNA probe wasprepared by reverse transcriptase of the mRNA from the sucrose gradientfraction from induced cells (arrow 23K of FIG. 1). The second probe wasprepared identically from the Similar fraction of the non-induced cellpreparation. Both cDNA probes were synthesized using the four highlyradioactive [³² P]-deoxynucleoside triphosphates as substrates andfragmented calf thymus DNA as primers. Random representation of the mRNAsequences in the cDNA probes was thereby achieved. Hybridization wascarried out at 62°-64° C. for 18 hours in 0.9M NaCl-0.09M Na citratebuffer pH 7.0, the initial colonies being hybridized with the cDNAprobes of the induced cells and the duplicate colonies with the cDNAprobes of the non-induced cells (or conversely) respectively. Afterextensive washing the filters were exposed to X-ray film and thebacterial colonies able to hybridize to the induced cDNA but not tonon-induced cDNA were identified. In this manner 20 different bacterialcolonies were isolated out of a total of over 2,000 transformed coloniesscreened. All of these 20 bacterial colonies contain multiple copies ofa plasmid in which were inserted sequences of human mRNA expressed onlyafter cells have been induced to produce interferon by poly (rI:rC).

An example illustrating this technique is shown in FIG. 2 in connectionwith fifteen pairs of alkali-treated pairs of colonies (initials andduplicates) on their nitrocellulose filters, whose DNA have beenhybridized with [³² P]-cDNA prepared against mRNA fraction 23K of FIG.1, from cells induced (i) or non-induced (n.i.) by poly(rI):(rC) forinterferon production. Arrows show two colonies, particularly coloniesnumbered 5 and 13, which contain induced sequences. Colony number 13 wasdesignated as E. coli DP50/A341. Clone A-341 was deposited on Jun. 2,1992, at Collection Nationale de Cultures de Microorganismes, InstitutPasteur, 25, rue de Docteur Roux, 75724 Paris Cedex 15, France, and hasbeen assigned depository accession number 1-1214.

c) Isolation of interferon mRNA from human fibroblast

Isolation of interferon mRNA (and demonstration of the presence ofinterferon cDNA sequences in the plasmid DNA of clone A341) wereobtained as follows: A 500 ml culture of this bacterial clone was usedto prepare 50 μg plasmid DNA. This DNA (after previous denaturation) wascovalently bound to diazobenzyloxymethyl cellulose pewder according tothe methods of Aldwine et al. (Proc.Natl.Acad. Sci. USA 1977, 74, 5350).In parallel, plasmid pBR322 DNA (not containing human DNA sequences) wassimilarly bound to cellulose. Poly A-containing mRNA, from humanfibroblasts induced to produce interferon, was hybridized to the two DNAcellulose preparations in 50% formamide at 52° C. and eluted by raisingthe formamide concentration to 100% at 70° C. The RNA recovered afterelution was translated in the reticulocyte cell-free system (FIG. 3).whereby the essential translation product of the mRNA selected on theA341 DNA-cellulose was found to be essentially the 23,000 MWpolypeptide. In contrast, no human interferon mRNA was recovered fromthe pBR322 DNA-cellulose. In comparison to the translation products ofthe human mRNA prior to hybridization to A341 DNA-cellulose it could beascertained that the cloned A341 DNA is complementary to only little ofthe mRNA of the mixture. The product of the mRNA selected on A341DNA-cellulose was immunoprecipitated by the anti-human fibroblastinterferon serum (see FIG. 3).

The interferon mRNA could also be isolated by a similar procedure tothat above but in which plasmid A341 DNA was bound to nitrocellulosefilters, the RNA hybridized to it, and eluted by boiling for 1 min in H₂O.

The activity of this purified mRNA to code for biologically potent humaninterferon has shown by injection to Xenopus laevis oocyte followed bymeasuring the inhibition of virus multiplication in human cells exposedto the oocyte incubation medium (Table 1). The interferon activity ofthe purified β2 mRNA was also shown the induction of (2'-5')oligo-isoadenylate synthetase in human cells by the oocyte translationproducts (Table 1).

Restriction enzyme mapping of A341 plasmid DNA showed that it contains ahuman DNA insert of about 900 nucleotides in the Pst site. The A341 DNAalso hybridized to 3 fragments of the human genome digested by Eco R1nuclease. These fragments are separated by agarose gel electrophoresis.Hybridization to agarose gel electrophoregrams of mRNA from humanfibroblast further showed that A341 DNA is complementary to RNAsequences that are expressed only in cells exposed to the interferoninducer poly (rI:rC),(FIG. 4a). Even a one hour exposure of the cells topoly(rI:rC)leads to the accumulation of a 1,250-1,350 nucleotide longRNA hybridizing to A341 DNA, which represents IFN-β2 mRNA.

The above data demonstrated that the bacterial clone E. coli DP50/A341contained in the Pst site of its pBR322 plasmid an insert of about 900nucleotides of human cDNA sequences which are complementary to a humaninterferon mRNA. Several similarly prepared clones were obtained. FIG.4b shows that clones for IFN-β2 hybridize to the largest 1,250-1,350nucleotide long mRNA while clones for IFN-β1 hybridized to the smallest900-1,000 nucleotide long mRNA.

The process can be used for obtaining clones of interferon DNA ofdifferent types (α, β, γ) from human cells.

Legends for Figures

FIG. 1: Sucrose gradient of poly A⁺ RNA from human cells induced toproduce interferon (a). Sedimentation was from right to left. TenXenopus laevis oocytes were injected with 0.4 μg RNA of each fractionand after 40 hours, the medium around the oocytes was assayed on FS11cells for interferon (left scale). Each RNA fraction (0.24 μg) wastranslated also in reticulocyte. lysates and the ³⁵ S-methionine labeledproducts were precipitated with anti-interferon serum. The productsanalyzed by polyacrylamide gel electrophoresis are shown in lane i of(b). Lane n in (b) represents the immunoprecipitated products ofunfractionated mRNA from non-induced cells. At the right end of (b) aremolecular weight markers (from top to bottom, 68, 46, 30, 18 and 14daltons×10⁻³). The position and intensity of the 23K and 20K protein(arrows) was recorded and is shown graphically in a (right scale). Theheaviest of the two interferon mRNAs is translated in the 23K proteinwhile the smallest interferon mRNA is translated in the 20K protein.

FIG. 2: Detection of transformed bacterial clone containing interferonDNA. Fifteen alkali-treated colonies on nitrocellulose filters, werehybridized with [³² P]cDNA prepared against the 23K mRNA fraction (seeFIG. 1) from cells induced (i) or non-induced (n.i.) by poly(rI):(rC)for interferon production. Arrows show two colonies which containinduced sequences. Colony number 13 is E. coli DP50/A341.

FIG. 3: Demonstration that clone E. coli DP50/A341 contains interferonDNA. Poly A⁺ mRNA from human fibroblast induced for interferonproduction was hybridized to DNA from A341 plasmid covalently bound tocellulose and translated. Gel electrophoresis of the translationproducts show that mRNA which codes for the interferon (IF) polypeptideis uniquely selected from the mixture of total mRNAs. pBR DNA is unableto select this mRNA. Position of IF polypeptide is shown afterimmuno-precipitation with anti-interferon (ipt).

FIG. 4: a) Plasmid DNA of bacterial clone A341 hybridizes to a 14S(1,300 nucleotides long) mRNA found in cells induced for interferonproduction (i) but not in non-induced cells (n). Plasmid pBR DNA doesnot hybridize while uncloned total cDNA (tot) hybridizes to many mRNAsfound also in non-induced cells. An agarose gel electrophoresis of theRNA followed by hybridization to the three ³² P-DNAs is shown.

b) Plasmid DNA from different clones of interferon DNA were used for asimilar experiment of hybridization to mRNA electrophoregrams. Clonescontaining IFN-β2 DNA hybridize to the 1,300 nucleotide long mRNA, whileclones with IFN-β1 DNA hybridize to the smaller 900 nucleotides longinterferon mRNA.

                  TABLE                                                           ______________________________________                                        HYBRIDIZATION-TRANSLATION OF                                                  β2-INTERFERON mRNA                                                                    Expt. 1   Expt. 2                                                             V.S.V.-   Oligo-isoadenylate                                                  Virus Yield                                                                             synthetase induction                                                                        Calcu-                                                Radio-    [.sup.32 P]-A2'p5'A, cpm                                                                    lated                                    Oocyte supernatant                                                                         immuno-   Oocyte extract                                                                              IF                                       (diluted 1:10)                                                                             assay, cpm.                                                                             (diluted 1:15)                                                                              titer                                    ______________________________________                                        uninjected   7445      1700                                                   with RNA hybridized                                                                        1920      4700          30                                       to IF-β2 plasmid                U/ml                                     with RNA hybridized                                                                        6015      1500           0                                       to unrelated plasmid                                                          Interferon standard                                                                         700      10,800                                                 100 U/ml                                                                      ______________________________________                                         *Clone E474 DNA was used in Expt. 1, and a pool of IFβ2 DNA plasmids     for Expt. 2.                                                             

We claim:
 1. An mRNA which consists essentially of an mRNA which istranslatable to human fibroblast interferon β2 polypeptide.
 2. A processfor obtaining an mRNA population which includes mRNAs containing thenucleotide sequence translatable to human interferon-β2, andsubstantially excluding mRNAs containing the nucleotide sequencetranslatable to human interferon-β1, comprising:a) cultivating humanfibroblast cells capable of producing fibroblast interferon-β whenexposed to an inducer of interferon; b) exposing said host cells to asaid inducer of interferon; c) extracting messenger RNAs from saidinduced host cells; and d) purifying said messenger RNAs to obtain thefraction thereof having about 1,250-1,350 nucleotides and correspondingto RNA sedimenting at 14S in sucrose gradient sedimentation.
 3. Aprocess in accordance with claim 2, wherein said purifying stepcomprises subjecting said mRNAs to sucrose gradient sedimentation andrecovering the RNAs sedimenting at 14S.
 4. A process for obtaining anmRNA population which includes mRNAs containing the nucleotide sequencetranslatable to human interferon-β1, and substantially excluding mRNAscontaining the nucleotide sequence translatable to human interferon-β2,comprising:a) cultivating human fibroblast host cells capable ofproducing fibroblast interferon-β when exposed to an inducer ofinterferon; b) exposing said host cells to a said inducer of interferon;c) extracting messenger RNAs from said induced host cells; and d)purifying said messenger RNAs to obtain the fraction thereof havingabout 900-1,000 nucleotides and corresponding to RNA sedimenting at 11Sin sucrose gradient sedimentation.
 5. A process in accordance with claim4, wherein said purifying step comprises subjecting said mRNAs tosucrose gradient sedimentation and recovering the RNA sedimenting at11S.